1. Field of the Invention
The present invention relates generally to a color cathode ray tube, and more particularly to a color cathode ray tube equipped with an in-line type electron gun and a deflection yoke which is designed to generate a uniform deflection magnetic field, thus constituting a dynamic convergence system capable of correcting a misconvergence of three electron beams in a vertical direction or a horizontal direction.
2. Description of the Prior Art
In general, a deflection yoke used in color cathode ray tubes equipped with an in-line type electron gun produces a pin-cushion shape horizontal deflection magnetic field and a barrel-shaped vertical deflection magnetic field. This type of deflection yoke is advantageous in eliminating the necessity of correcting deflections by use of a dynamic convergence system, thereby requiring no complex convergence circuit. However, the non-uniform deflection magnetic field is likely to distort the beam spots owing to deflection, thereby failing to achieve high resolution particularly in a peripheral area of a picture image.
It is known that a cathode ray tube equipped with a delta type electron gun is provided with a deflection yoke capable of generating a uniform magnetic field. To overcome the problem pointed out above, the same method is applied to a color cathode ray tube equipped with an in-line type electron gun should be provided with such a deflection yoke so as to constitute a dynamic convergence system capable of correcting a misconvergence of three electron beams.
Referring to FIG. 5, Japanese Laid-Open Patent Publication No. 64-62993 discloses a color cathode ray tube having a convergence unit 1 provided at the head of an anode which acts as a final accelerating grid, and a pair of mutually opposing magnetic field generators 2 and 3 mounted on the periphery of a neck portion 4 of the cathode ray tube. The convergence unit 1 includes two pairs of magnetic pole pieces 7a, 7b and 8a, 8b in a cup-shaped housing of non-magnetic metal. The pole pieces 7a, 7b and 8a, 8b are disposed with electron beam paths 5 and 6 being interposed between each pair. In addition, a center beam path 9 is disposed between a pair of shield plates 9a and 9b. The magnetic field generators 2 and 3 include U-shaped cores 2a and 3a around which coils 2b and 3b are wound, and are supported so that their leg portions are magnetically connected to the outsides of the pole pieces 7a, 7 b, and 8a, 8b, respectively.
The dynamic convergence correction is effected by passing a parabolic current or any other sawtooth-shaped waveform dynamic current through the coils 2b and 3b, wherein the dynamic current increases proportionally as the degree of beam deflection becomes large. The electron beams are deflected in the opposite direction, as indicated by the large arrows 1Oa and 10b, owing to the two-pole magnetic fields generated from the legs of the cores 2a and 3a to the pole pieces 7a, 7b and 8a, 8b.
An advantage of the dynamic convergence correction is that the corrected magnetic field is intensified by the two pairs of pole pieces, thereby ensuring an optimum convergence correction with high sensitivity. The disadvantage is that the dynamic convergence correction is only effected in a horizontal direction and cannot be effected in a vertical and a horizontal direction at the same time.